Techniques for rach (random access channel)-less synchronized handover for wireless networks

ABSTRACT

An example technique is provided for receiving, by a target base station (BS) from a source BS, information identifying a source cell or the source BS, and a first time advance value used by the user device to transmit signals to the source BS, receiving a signal by the target BS that was transmitted from the user device based on the first time advance value, determining, by the target BS based upon the first time advance value and the received signal from the user device, a second time advance value to be used by the user device to transmit data to the target BS, sending the second time advance value from the target BS to the source BS, receiving, by the target BS, a handover of the user device from the source BS to the target BS, and receiving data by the target BS from the user device based on the second time advance value.

TECHNICAL FIELD

This description relates to wireless networks.

BACKGROUND

A communication system may be a facility that enables communicationbetween two or more nodes or devices, such as fixed or mobilecommunication devices. Signals can be carried on wired or wirelesscarriers.

An example of a cellular communication system is an architecture that isbeing standardized by the 3rd Generation Partnership Project (3GPP). Arecent development in this field is often referred to as the long-termevolution (LTE) of the Universal Mobile Telecommunications System (UMTS)radio-access technology. E-UTRA (evolved UMTS Terrestrial Radio Access)is the air interface of 3GPP's Long Term Evolution (LTE) upgrade pathfor mobile networks. In LTE, base stations, which are referred to asenhanced Node Bs (eNBs), provide wireless access within a coverage areaor cell. In LTE, mobile devices, or mobile stations are referred to asuser equipments (UE). LTE has included a number of improvements ordevelopments. A mobile station or UE may perform a handover from asource BS to a target BS. In many technologies, a handover may use arandom access procedure to allow the MS to obtain a time advance valuewith respect to the target BS. Also, a handover often causes aninterruption in data services for a MS.

SUMMARY

According to an example implementation, a method includes performing asynchronized and random access procedure-less handover including:receiving, by the user device from the source BS, a handover commandincluding at least a handover time field that identifies a time toperform a synchronized handover to the target BS, performing, by theuser device without using a random access procedure, a handover from thesource BS to the target BS at the time identified by the handover timefield, and sending a handover completion message from the user device tothe target BS.

In an example implementation, the performing a handover may include theuser device: beginning to receive data from the target BS atapproximately the time identified by the handover time field, andceasing to receive data from the source BS at approximately the timeidentified by the handover time field.

In an example implementation, the method may further includedetermining, by the user device in an autonomous and a random accessprocedure-free manner, a time advance value for the user device totransmit data to the target BS.

In an example implementation, the determining the time advance value forthe user device to transmit data to the target BS may include:determining, by the user device, a first propagation delay from thesource BS to the user device, determining, by the user device, a timedifference value as a difference between the first propagation delay anda second propagation delay, the second propagation delay including apropagation delay from a target BS to the user device, determining, bythe user device based on the first propagation delay and the timedifference value, a second time advance value for use in transmittingsignals from the user device to the target BS.

In an example implementation, the method may include receiving, by theuser device from the target BS, within an initial scheduling grant fromthe target BS after handover has been performed to the target BS, a timeadvance value for the user device to transmit data to the target BS.

In an example implementation, the method may further include receiving,by the user device from the source BS within the handover command, atime advance value for the user device to transmit data to the targetBS.

In an example implementation, the method may further include the userdevice transmitting data to the target BS using a shortened packet ortransmission time interval (TTI), including omitting one or more symbolsin the shortened packet or TTI based on a coverage area or transmissionpower of the target BS.

In an example implementation, an apparatus includes at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: receive,by the user device from the source BS, a handover command including atleast a handover time field that identifies a time to perform asynchronized handover to the target BS, perform, by the user devicewithout using a random access procedure, a handover from the source BSto the target BS at the time identified by the handover time field, andsend a handover completion message from the user device to the targetBS.

In an example implementation, a computer program product includes anon-transitory computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: receiving, by the user device from thesource BS, a handover command including at least a handover time fieldthat identifies a time to perform a synchronized handover to the targetBS, performing, by the user device without using a random accessprocedure, a handover from the source BS to the target BS at the timeidentified by the handover time field, and sending a handover completionmessage from the user device to the target BS.

In an example implementation, a method includes determining, by a userdevice, a first time advance value for the user device for use intransmitting signals from the user device to a source base station(source BS), determining, by the user device, a first propagation delayfrom a source base station (BS) to the user device, determining, by theuser device, a time difference value as a difference between the firstpropagation delay and a second propagation delay, the second propagationdelay including a propagation delay from a target BS to the user device,determining, by the user device based on the first propagation delay andthe time difference value, a second time advance value for use intransmitting signals from the user device to the target BS, performing,by the user device, a handover from the source BS to the target BS, andtransmitting signals from the user device to the target BS based uponthe second time advance value.

In an example implementation, the performing a handover may includeperforming a synchronized and random access-free handover from thesource BS to the target BS in response to a handover command receivedfrom the source BS, the synchronized handover being performed by theuser device at a time identified in the handover command.

In an example implementation, the performing a handover from the sourceBS to the target BS may include: sending, from the user device to thesource BS, a measurement report indicating a handover to the target BS,receiving, by the user device from the source BS, a handover commandincluding at least a handover time field that identifies a time toperform the handover to the target BS, beginning to receive data fromthe target BS at a time identified by the handover time field, andsending a handover completion message from the user device to the targetBS.

In an example implementation, the determining the second time advancevalue may include determining the second time advance value based on thefollowing:

TA2=TA1−2*X, wherein TA1 is the first time advance value, TA2 is thesecond time advance value, and X is the time difference value.

In an example implementation, the determining the time difference valuemay include the user device performing the following: receiving a firstsignal from the source BS, receiving a second signal from the target BS,wherein the first signal and the second signal are transmitted at thesame time, determining a difference in an arrival time between the firstsignal and the second signal as received by the user device, wherein thetime difference value comprises the difference in arrival time of thefirst signal and the second signal.

In an example implementation, the determining the time difference valuemay include the user device performing the following: receiving a firstcommon reference signal(s) (CRS) from the source BS, receiving a secondCRS from the target BS, wherein the first CRS and the second CRS aretransmitted at the same time, and determining a difference in an arrivaltime between the first CRS and the second CRS as received by the userdevice, wherein the time difference value comprises the difference inarrival time of the first CRS and the second CRS.

In an example implementation, an apparatus may include at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to:determine, by a user device, a first time advance value for the userdevice for use in transmitting signals from the user device to a sourcebase station (source BS), determine, by the user device, a firstpropagation delay from a source base station (BS) and the user device,determine, by the user device, a time difference value as a differencebetween the first propagation delay and a second propagation delay, thesecond propagation delay including a propagation delay from a target BSand the user device, determine, by the user device based on the firstpropagation delay and the time difference value, a second time advancevalue for use in transmitting signals from the user device to the targetBS, perform, by the user device, a handover from the source BS to thetarget BS, and transmit signals from the user device to the target BSbased upon the second time advance value.

In an example implementation, a computer program product includes anon-transitory computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: determining, by a user device, a first timeadvance value for the user device for use in transmitting signals fromthe user device to a source base station (source BS), determining, bythe user device, a first propagation delay from a source base station(BS) and the user device, determining, by the user device, a timedifference value as a difference between the first propagation delay anda second propagation delay, the second propagation delay including apropagation delay from a target BS and the user device, determining, bythe user device based on the first propagation delay and the timedifference value, a second time advance value for use in transmittingsignals from the user device to the target BS, performing, by the userdevice, a handover from the source BS to the target BS, and transmittingsignals from the user device to the target BS based upon the second timeadvance value.

In an example implementation, a method may include receiving, by atarget base station (BS) from a source BS, information identifying auser device that is connected to the source BS, information identifyinga source cell or the source BS, and a first time advance value used bythe user device to transmit signals to the source BS, receiving a signalby the target BS that was transmitted from the user device based on thefirst time advance value, determining, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS, sending the second time advance value from the targetBS to the user device, receiving a handover of the user device from thesource BS to the target BS, receiving data by the target BS from theuser device based on the second time advance value.

In an example implementation, the method may further include receivingby the target BS from the source BS a handover request to perform ahandover of the user device from the source BS to the target BS, thehandover request including a handover time that identifies a time when asynchronized handover for the user device is to be performed from thesource BS to the target BS.

In an example implementation, the method may further include receiving,by the target BS from the source BS, a signal configuration informationidentifying a configuration of a reference signal transmitted by theuser device to the source BS using the first time advance value,receiving, by the target BS from the source BS, and schedulinginformation identifying scheduled resources for the user device totransmit the reference signal to the source BS, wherein the receiving asignal by the target BS from the user device includes the target BSreceiving the scheduled reference signal from the user device.

In an example implementation, the determining may include determining,by the target BS, a second time advance value to be used by the userdevice to transmit data to the target BS based upon the first timeadvance value and the reference signal received from the user device viathe scheduled resources.

In an example implementation, the reference signal may include asounding reference signal.

In an example implementation, the receiving a handover may includereceiving a synchronized and random access-free handover of the userdevice from the source BS to the target BS in response, the synchronizedhandover being performed at a time identified by a handover command.

In an example implementation, a computer program product, the computerprogram product comprising a non-transitory computer-readable storagemedium and storing executable code that, when executed by at least onedata processing apparatus, is configured to cause the at least one dataprocessing apparatus to perform a method including: receiving, by atarget base station (BS) from a source BS, information identifying auser device that is connected to the source BS, information identifyinga source cell or the source BS, and a first time advance value used bythe user device to transmit signals to the source BS, receiving a signalby the target BS that was transmitted from the user device based on thefirst time advance value, determining, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS, sending the second time advance value from the targetBS to the user device, receiving a handover of the user device from thesource BS to the target BS, and receiving data by the target BS from theuser device based on the second time advance value.

In an example implementation, an apparatus includes at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: receive,by a target base station (BS) from a source BS, information identifyinga user device that is connected to the source BS, informationidentifying a source cell or the source BS, and a first time advancevalue used by the user device to transmit signals to the source BS;

receive a signal by the target BS that was transmitted from the userdevice based on the first time advance value, determine, by the targetBS based upon the first time advance value and the received signal fromthe user device, a second time advance value to be used by the userdevice to transmit data to the target BS, send the second time advancevalue from the target BS to the user device, receive a handover of theuser device from the source BS to the target BS, and receive data by thetarget BS from the user device based on the second time advance value.

In an example implementation, a method includes receiving, by a targetbase station (BS) from a source BS, information identifying a userdevice that is connected to the source BS, information identifying asource cell or the source BS, and a first time advance value used by theuser device to transmit signals to the source BS, receiving a signal bythe target BS that was transmitted from the user device based on thefirst time advance value, determining, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS, sending the second time advance value from the targetBS to the source BS, receiving, by the target BS, a handover of the userdevice from the source BS to the target BS, and receiving data by thetarget BS from the user device based on the second time advance value.

In an example implementation, the source BS sends the second timeadvance value to the user device.

In an example implementation, the method may further include the sourceBS sending a handover command to the user device, the handover commandidentifying a synchronized and random access procedure-free handover tothe target BS, a time to perform the handover, and the second timeadvance value.

In an example implementation, the method may further include receivingby the target BS from the source BS a handover request to perform ahandover of the user device from the source BS to the target BS, thehandover request including a handover time that identifies a time when asynchronized handover for the user device is to be performed from thesource BS to the target BS, and sending, by the target BS to the sourceBS, a handover request acknowledgement that acknowledges that asynchronized handover will be performed for the user device from thesource BS to the target BS at the identified time.

In an example implementation, the method may further include receiving,by the target BS from the source BS, a signal configuration informationidentifying a configuration of a reference signal transmitted by theuser device to the source BS using the first time advance value,receiving, by the target BS from the source BS, scheduling informationidentifying scheduled resources for the user device to transmit thereference signal to the source BS, wherein the receiving a signal by thetarget BS from the user device includes the target BS receiving thescheduled reference signal from the user device.

In an example implementation, the determining may include determining,by the target BS, a second time advance value to be used by the userdevice to transmit data to the target BS based upon the first timeadvance value and the reference signal received from the user device viathe scheduled resources.

In an example implementation, the reference signal may include asounding reference signal.

In an example implementation, the receiving a handover may includereceiving a synchronized and random access-free handover of the userdevice from the source BS to the target BS, the synchronized handoverbeing performed at a time identified by a handover command.

In an example implementation, a computer program product may include anon-transitory computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: receiving, by a target base station (BS)from a source BS, information identifying a user device that isconnected to the source BS, information identifying a source cell or thesource BS, and a first time advance value used by the user device totransmit signals to the source BS, receiving a signal by the target BSthat was transmitted from the user device based on the first timeadvance value, determining, by the target BS based upon the first timeadvance value and the received signal from the user device, a secondtime advance value to be used by the user device to transmit data to thetarget BS, sending the second time advance value from the target BS tothe source BS, receiving, by the target BS, a handover of the userdevice from the source BS to the target BS, and receiving data by thetarget BS from the user device based on the second time advance value.

In an example implementation, an apparatus may include at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: receive,by a target base station (BS) from a source BS, information identifyinga user device that is connected to the source BS, informationidentifying a source cell or the source BS, and a first time advancevalue used by the user device to transmit signals to the source BS,receive a signal by the target BS that was transmitted from the userdevice based on the first time advance value, determine, by the targetBS based upon the first time advance value and the received signal fromthe user device, a second time advance value to be used by the userdevice to transmit data to the target BS, send the second time advancevalue from the target BS to the source BS, receiving, by the target BS,a handover of the user device from the source BS to the target BS, andreceive data by the target BS from the user device based on the secondtime advance value.

In an example implementation, a method may include determining, by asource base station (BS), a first time advance value for the user deviceto use in transmitting signals from the user device to source BS,sending, from the source BS to the user device a request to measure atarget BS propagation delay of a signal transmitted from a target BS tothe user device, receiving the target BS propagation delay from the userdevice, determining, by the source BS, a second time advance value forthe user device to use in transmitting signals to the target BS, andsending a handover command to the user device, the handover commandincluding the second time advance value.

In an example implementation, the determining, by the source BS, thesecond time advance value for the user device may include determining,by the source BS based on the first time advance value and the target BSpropagation delay, the second time advance value for the user device touse in transmitting signals to the target BS.

In an example implementation, the determining, by the source BS, thesecond time advance value for the user device may include determining,by the source BS based on the following: TA2=TA1−2*(T2−T1), where T1 isa source BS propagation delay of a signal transmitted from the source BSto the user device, T2 is the target BS propagation delay, TA1 is thefirst time advance value for the user device, and TA2 is the second timeadvance value for the user device.

In an example implementation, the method further includes performing asynchronized and random access procedure-free handover of the userdevice from the source BS to the target BS based on the handovercommand, the handover of the user device being performed at a timeidentified in the handover command.

In an example implementation, the method may further include performinga handover of the user device from the source BS to the target BS,including: receiving, by the source BS from the user device, ameasurement report indicating a handover to the target BS, and sending,by the source BS to the user device, a handover command including atleast a handover time field that identifies a time to perform a randomaccess procedure-free and synchronized handover to the target BS.

In an example implementation, a computer program product includes anon-transitory computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: determining, by a source base station (BS),a first time advance value for the user device to use in transmittingsignals from the user device to source BS, sending, from the source BSto the user device a request to measure a target BS propagation delay ofa signal transmitted from a target BS to the user device, receiving thetarget BS propagation delay from the user device, determining, by thesource BS, a second time advance value for the user device to use intransmitting signals to the target BS, and sending a handover command tothe user device, the handover command including the second time advancevalue.

In an example implementation, an apparatus includes at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to:determine, by a source base station (BS), a first time advance value forthe user device to use in transmitting signals from the user device tosource BS, send, from the source BS to the user device a request tomeasure a target BS propagation delay of a signal transmitted from atarget BS to the user device, receive the target BS propagation delayfrom the user device, determine, by the source BS, a second time advancevalue for the user device to use in transmitting signals to the targetBS, and send a handover command to the user device, the handover commandincluding the second time advance value.

In an example implementation, a method includes sending, from a sourcebase station (BS) to a target BS, a request for expected coverage rangeor transmission power level for a cell associated with the target BS,receiving, by the source BS from the target BS, the expected coveragerange or transmission power level of the cell associated with the targetBS, sending the expected coverage range or transmission power level ofthe cell associated with the target BS from the source BS to a userdevice, sending a handover command to the user device to cause ahandover for the user device from the source BS to the target BS,wherein the user device omits one or more symbols from one or morepackets transmitted by the user device to the target BS based on theexpected coverage range or transmission power level of the target BS.

In an example implementation, the user device omitting may include theuser device omitting a first number of symbols of one or more packets ifthe expected coverage range or transmission power is above a firstthreshold and less than a second threshold, and the user device omittinga second number of symbols of one or more packets if the expected coverrange or transmission power is greater than the second threshold,wherein the second number is greater than the first number.

In an example implementation, the user device omitting may include theuser device: omitting three symbols of one or more packets if theexpected coverage range or transmission power indicates a macro cell orlarge cell, omitting two symbols of one or more packets if the expectedcoverage range or transmission power indicates a micro cell or mediumcell, where the micro cell is smaller than the macro cell, and omittingone symbol of one or more packets if the expected coverage range ortransmission power indicates a pico cell or small cell, where the picocell is smaller than the micro cell.

In an example implementation, a computer program product includes anon-transitory computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: sending, from a source base station (BS) toa target BS, a request for expected coverage range or transmission powerlevel for a cell associated with the target BS, receiving, by the sourceBS from the target BS, the expected coverage range or transmission powerlevel of the cell associated with the target BS, sending the expectedcoverage range or transmission power level of the cell associated withthe target BS from the source BS to a user device, sending a handovercommand to the user device to cause a handover for the user device fromthe source BS to the target BS, wherein the user device omits one ormore symbols from one or more packets transmitted by the user device tothe target BS based on the expected coverage range or transmission powerlevel of the target BS.

In an example implementation, an apparatus includes at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: send,from a source base station (BS) to a target BS, a request for expectedcoverage range or transmission power level for a cell associated withthe target BS, receive, by the source BS from the target BS, theexpected coverage range or transmission power level of the cellassociated with the target BS. send the expected coverage range ortransmission power level of the cell associated with the target BS fromthe source BS to a user device, sending a handover command to the userdevice to cause a handover for the user device from the source BS to thetarget BS, wherein the user device omits one or more symbols from one ormore packets transmitted by the user device to the target BS based onthe expected coverage range or transmission power level of the targetBS.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a handover of a user device froma source base station (BS) to a target BS according to an exampleimplementation.

FIG. 2 is a timing diagram that illustrates an example unsynchronizedhandover where a user device uses a random access procedure to obtain atime advance value.

FIG. 3 is a diagram illustrating an example of a general synchronizedRACH-less (or random access procedure-less) handover procedure accordingto an example implementation.

FIG. 4 is a diagram illustrating downlink data scheduling from a sourceBS and a target BS before and after a handover time 410 according to anexample implementation.

FIG. 5 is a diagram illustrating downlink data scheduling from a sourceBS and a target BS before and after a handover time 410, including ahysteresis time 510, according to an example implementation.

FIG. 6 is a diagram illustrating a second example implementation inwhich a target BS estimates a time advance value for a user device withrespect to the target BS.

FIG. 7 is a diagram illustrating a third example implementation in whicha source BS determines a time advance value for a user device withrespect to a target BS.

FIG. 8 is a diagram illustrating decreasing a size of a packet accordingto an example implementation.

FIG. 9 is a diagram illustrating a decreasing of a size of a packet toavoid interference with an adjacent packet.

FIG. 10 is a diagram illustrating a fifth example implementation inwhich a target BS estimates a time advance value for a user device withrespect to the target BS, and then forwards the time advance value tothe source BS for forwarding to the user device.

FIG. 11 is a flow chart illustrating operation of a user deviceaccording to an example implementation.

FIG. 12 is a flow chart illustrating operation of a user deviceaccording to another example implementation.

FIG. 13 is a flow chart illustrating operation of a target base stationaccording to an example implementation.

FIG. 14 is a flow chart illustrating operation of a target base stationaccording to another example implementation.

FIG. 15 is a flow chart illustrating operation of a source base stationaccording to an example implementation.

FIG. 16 is a flow chart illustrating operation of a source base stationaccording to an example implementation.

FIG. 17 is a block diagram of a wireless station (e.g., a BS or a userdevice, or other communications device) 1700 according to an exampleimplementation.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating a handover of a user device froma source base station (BS) to a target BS according to an exampleimplementation. A source BS 132 may provide wireless services withinsource cell 130 to one or more wireless user devices. A target BS 142may provide wireless services within a target cell 140 to one or morewireless user devices. Multiple user devices or user equipments (UEs),including user device 134, may be connected (and in communication) withBS 132, for example. BSs may also be referred to as enhanced Node Bs(eNBs) in the case where the radio access technology (RAT) is LTE, butmay have other names for other RATs. A plurality of user devices (notshown) may be connected to and in communication with target BS 142.While only one user device is shown (user device 134), multiple userdevices may be connected (and in communication) with each BS.

According to an example implementation, at least part of thefunctionalities of a base station (BS) or (e) Node B may be also becarried out by any node, server or host which may be operably coupled toa transceiver, such as a remote radio head. Although not shown, each ofthe BSs may be connected together via one or more communication links,such as a wired link or a wireless communications link. One example ofsuch communication link could be the X2 interface. Also, one or more ofthe BSs may also be connected to a core network, e.g., via a S1interface or other interface. This is merely one simple example of awireless network, and others may be used. In one example implementation,source BS 132 and target BS 142 may have the same time synchronization,e.g., having or using a same clock for synchronization. In anotherexample implementation, both source BS 132 and target BS 142 may besynchronized based on a same clock, but an offset in timing may existbetween the source BS and target BS, as examples.

A user device (user terminal, user equipment (UE)) may refer to astationary (e.g., non-portable) computing device or a portable computingdevice that includes wireless mobile communication devices operatingwith or without a subscriber identification module (SIM), including, butnot limited to, the following types of devices: a mobile station, amobile phone, a cell phone, a smartphone, a personal digital assistant(PDA), a handset, a device using a wireless modem (alarm or measurementdevice, a patient medical monitoring device, etc.), a laptop and/ortouch screen computer, a tablet, a phablet, a game console, a notebook,and a multimedia device, as examples. It should be appreciated that auser device may also be a nearly exclusive uplink only device, of whichan example is a camera or video camera loading images or video clips toa network. A user device may also include downlink-only devices, ordevices that primarily receive downlink communications, such as a pager.

User device 132 may initially be connected to source BS 132. Eventually,e.g., based on movement of user device 134 towards BS 142, BS 142 maybecome a better serving BS than BS 132, e.g., based on a greater signalstrength from BS 142 as compared to BS 132, as measured by user device134. Based on the measurement of signals transmitted by multiple BSs(including signals transmitted by BS 132 and BS 142), user device 134may send a measurement report to source BS 132, indicating a handover(HO) to the target BS 142 (e.g., based on signal strength). This maycause a handover procedure to be performed to handover the user device134 from the source BS 132 to the target BS 142. The handover proceduremay typically involve the user device 134 performing a random accessprocedure with the target BS to obtain a time advance value.

FIG. 2 is a diagram that illustrates an example unsynchronized handoverwhere a user device uses a random access procedure to obtain a timeadvance value. A handover of user device 134 from a source BS 132 to atarget BS 142 may, for example, be triggered by a measurement report at210 sent from user device 134 to source BS 132 indicating handover totarget BS 142. For example, a RRC (Radio Resource Control) measurementreport may be triggered to be sent when one of several conditions hasoccurred (or one of several signal conditions has been detected by theuser device/UE), such as, for example: 1) a signal received from aneighbor BS has a greater received signal strength than a signalreceived from the serving cell/source BS (e.g., indicating that betterperformance may be obtained by the user device performing a handover andconnecting to the neighbor cell/BS); or, 2) a signal from aprimary/serving cell (or source BS) becomes worse (e.g., a receivedsignal strength as measured by user device is less) than a definedthreshold, and a signal from a neighbor cell becomes some offset better(e.g., signal strength is greater) than a signal from theprimary/serving cell. These are merely some examples, and other signalconditions may be used to trigger or cause the user device or UE totransmit a RRC measurement report to a serving cell/source BS, forexample. In an example implementation, a measurement report may indicatea handover to a target BS when, for example, the measurement reportindicates that a received signal strength from the target BS (asmeasured by the user device) is an offset better than the receivedsignal strength of the serving cell/source BS. This is merely anexample.

After a last transmission of a packet at 212 from source BS 132 to userdevice 134, the source BS 132 may send a handover command, such as aRadio Resource Connection (RRC) Connection Reconfiguration at 214, touser device 134, indicating handover to target BS 142, includinginformation for target BS 142. The handover command or RRC ConnectionReconfiguration may include a random access preamble. Next, the userdevice 134 and the target BS 132 may perform a random access procedurevia a random access channel (RACH), including the user device 134 (orUE) sending a random access request including the random access preamblevia the RACH to the target BS 142. The target BS 142 may determine atime advance value for the user device 134 based on the propagationdelay of the transmitted preamble. The target BS 142 may send a randomaccess response to the user device 134 that includes the time advancevalue that may be used by the user device 134 to adjust the timing ofits transmission to the target BS so the user device 134 may transmituplink to the target BS 142 in a synchronized manner. After the randomaccess procedure at 216, the user device 134 may send a RRC ConnectionReconfiguration complete (or handover complete message) to the target BS142. And, the user device 134 may receive system information from thetarget BS 142 at 219, and may receive data packets from the target BS at220. Thus, in this example, the handover of the user device 134 may beconsidered unsynchronized because a significant delay exists between atime (at 212) the user device 134 stops receiving data from the sourceBS 132 and a time (at 220) the user device 134 starts receiving datafrom the target BS 142. Therefore, in this example shown in FIG. 2 of anunsynchronized handover, the total interruption time 224 (includinghandover time 222) may be significant, e.g., which may include a delayfrom a last data transmission from the source BS 132 at 212 until afirst data transmission from the target BS to the user device 134 at220, for example.

Therefore, based on the example handover shown in FIG. 2, in some cases,a handover procedure may involve both an interruption time (224) with nodata connectivity for the user device 134 as well as overhead from therandom access procedure with the target BS 142/target cell 140.According to an example implementation, various example implementationsare described to perform a random access procedure-less (or RACH-less)and time-synchronized (or simply synchronized) handover for a userdevice from a source BS 132 to a target BS 142, e.g., in order to reduceoverhead and decrease the interruption time 224 for the user device.

A random access procedure-less handover may be or may include a handoverwhere the user device does not acquire a time advance value via a randomaccess procedure (e.g., no random access procedure is used). In severaldifferent example implementations, the user device 134 may acquire atime advance value for the target BS 142 via another technique (e.g.,without using a random access procedure). A synchronized handover mayinclude a handover for a user device where the user device may beginreceiving data within X ms of the user device receiving a last data fromthe source BS 132 prior to handover, where X is a smaller number thanprovided by use of a random access procedure. For example, the totalinterruption time 224 may be, 50-150 ms, and may be on the order of 100ms, as an example. For an example of a synchronized handover, the userdevice may begin receiving data from the target BS 142 at approximatelythe same time as the user device 134 last receives data from the sourceBS 132, where approximately may mean, for example, that the interruptiondelay 224 for a synchronized handover may be, for example, less than 2ms, less than 5 ms, or less than 10 ms, as examples.

FIG. 3 is a diagram illustrating an example of a general synchronizedRACH-less (or random access procedure-less) handover procedure accordingto an example implementation. At operation 1) of FIG. 3, the user devicemeasures signals transmitted by various BSs, including signalstransmitted by source BS 132 and target BS 142, such as, for example,measuring common reference signals (CRS) signals. The user device 134may determine that target BS 142 provides greater signal strength ascompared to source BS 132. Therefore, at operation 1), user device 134may send a radio resource measurement report to source BS 132 indicatinghandover to target BS 142. This measurement reports triggers a handoverof the user device 134 to target BS 142.

At operation 2) of FIG. 3, assuming that the source cell or source BS142 decides to initiate handover, the source BS 132 informs the targetcell or target BS 142 of the requested handover via the X2 interface,including informing the target cell of the time when the handover willbe performed. For example, source BS 132 may send a handover request totarget BS 142, identifying a time for synchronized handover for userdevice 134 from source BS 132 to target BS 142.

At operation 3) of FIG. 3, the target BS 142 confirms the handover tothe source BS 132, including the handover time. For example, target BS142 may send a handover request acknowledgement to source BS 132 toconfirm handover of the user device 134 to target BS 142 at theidentified/requested handover time. If the handover time proposed by thesource BS 132 is unacceptable to the target BS 142, then target BS 142may send a handover request acknowledgement which includes a newproposed time for synchronized handover. Note that operations 2) and 3)of FIG. 3 may be repeated, as necessary, so that source BS 132 andtarget BS 142 may negotiate and/or agree on a time for the synchronizedhandover.

At operation 4) of FIG. 3, the source BS 132 sends a handover command(e.g., a RRC connection reconfiguration) to the user device 134,informing the user device 134 of the time (e.g., exact time-instant)where it should perform the handover from source cell/source BS 132 tothe target cell/target BS 132. The handover command may include, forexample, a field indicating handover, information identifying the targetBS 142 or target cell (e.g., BSID or cell ID), information (e.g., userdevice ID or cell radio network temporary identifier/C-RNTI for the userdevice or UE) identifying the user device 134 to which the handovercommand is directed, and the time for the synchronized handover. Thetime for handover may be indicated in seconds, by frame number, or othervalues or quantities that may indicate a specific time for a handover.In an example implementation, this identified time for handover may bethe time which the user device 134 stops receiving or listening totransmissions from the source cell/source BS 132 and starts receiving orlistening to transmissions from the target cell/target BS 142 stopstransmitting data to the user device 134 on or before the handover time.

At operation 5), at the time for handover indicated in the handovercommand, the target cell/target BS 142 schedules DL data fortransmission to the user device 134 (e.g., user device 134 may beginreceiving data from the target BS 142 at or approximately the time forhandover, for example), and the source cell/source BS 132 stopstransmitting data to the user device 134 at (or possibly before) thishandover time. At operation 6) of FIG. 3, the user device 134 sends ahandover complete message (e.g., RRC connection reconfigurationcomplete) to the target cell/target BS 142 to indicate that the handoverto the target cell/target BS 142 is completed.

FIG. 4 is a diagram illustrating downlink data scheduling from a sourceBS and a target BS before and after a handover time 410 according to anexample implementation. Scheduled downlink data 412 (shown in grey) fromthe source BS 132 is shown in a top row, and scheduled downlink data 414(shown in grey) from the target BS 142 is shown in a bottom row. Thedata is divided into transmission packets, where, for example, eachpacket may include one or more transmission time intervals or TTIs. Ahandover time 410 is shown between TTIs (and could also be shown betweenpackets or frames). In this example implementation, the source BS 132has scheduled transmission of data or transmits data to the user device134 at downlink data 412 before the handover time 410 (also referred toas handover switch time). While target BS 142 transmits data (or hasscheduled the transmission of data) to user device 134 at downlink data414 after the handover time 410. Thus, the user device 134 may listen to(e.g., receive and decode) data 412 before the handover time 410, andmay listen to (e.g., receive and decode) data 414 from the target BSafter handover time 410. Although the transmission from the source BS132 and target BS 142 are time synchronized, there might be a minoroffset 416 in the timing of the source cell/source BS 132 and targetcell/target BS 142. Based on this offset 416 in timing between source BS132 and target BS 142, there may be an interruption time period (e.g.,corresponding to offset 416) that may be a fraction or portion of a TTIor a portion or fraction of a packet, for example.

FIG. 5 is a diagram illustrating downlink data scheduling from a sourceBS and a target BS before and after a handover time 410, including ahysteresis time 510, according to an example implementation. In theexample implementation shown in FIG. 5, there is a period of time(hysteresis time 510) where the user device or UE may listen to (orreceive and decode) signals from both source BS 132 and target BS 142.The advantage of this example implementation, is that the source BS 132will be more likely (higher probability) to transmit any pending HybridARQ (HARQ) packet retransmissions (e.g., data that was not acknowledgedas being received) to the user device 134 (and other user devices)during such hysteresis time 510. Both the source BS 132 and the targetBS 142 will know or receive the hysteresis time 510 (the length of thehysteresis time 510). The length of the hysteresis time 510 may befixed, or may be negotiated between source BS and target BS withinoperations 2), 3) and 4) of FIG. 3. The cost for this hysteresis timemay include slightly more complexity at the source BS and target BS andthe user device, and usage of slightly more resources during thehysteresis time 510.

As noted above with respect to FIG. 3, a RACH-less (random accessprocedure-less) handover may be performed. A number of different exampleimplementations are described below for RACH-less techniques to eithercalculate a time advance value (without use of random access procedure)for a user device 134 with respect to a target BS 142 or to temporarilyreduce a length of an uplink transmission packet to a target BS when atime advance value is unavailable, in order to stay within atransmission time interval for a packet or frame or other chunk of data.

A first example implementation may include a user device performing anautonomous calculation of a target cell/target BS time advance value(TA).

When calculating the time advance value towards the target cell, theuser device or UE may use the assumption that the network issynchronized and operate according to the following assumptions andcalculations, as an example:

Propagation delay from source cell/source BS 132 to user device 134=T1;

Propagation delay from target cell/target BS 142 to user device=T2;

Current time advance value towards source cell/source BS 132=TA1=2*T1.In other words, a time advance value for a user device or UE may be setto twice the propagation delay with respect to such BS. Thus, both TA1and T1 are known by user device. Also, the user device 134 is able tomeasure a time difference in signals received from the sourcecell/source BS 132 and the target cell/target BS 142, since the userdevice 134 is capable of measuring radio link measurements.

The user device 134 will measure a time difference value: X=T1−T2. Forexample, the user device 134 may measure a time difference of commonreference signals (CRS) transmitted by the source BS 132 and target BS142. The CRS signals may typically be transmitted at the same time,based on BSs being synchronized, for example.

Time advance value towards the target cell would be: TA2=2*T2.

Following this, the time advance value towards the target cell may becalculated based on the following, for example:

TA2=2*(T1−X)=TA1−2*X.

Therefore, according to an example implementation, a user device maycalculate or determine a time advance value towards (or with respect to)source BS 142 based on the following method or technique: Determining,by a user device, a first time advance value (TA1) for the user devicefor use in transmitting signals from the user device to a source BS.Determining, by the user device, a first propagation delay (T1) from thesource BS and the user device (e.g., T1=TA1/2). Determining, by the userdevice, a time difference value (X) as a difference between the firstpropagation delay (T1) and a second propagation delay (T2), the secondpropagation delay including a propagation delay from a target BS to theuser device. Determining, by the user device based on the firstpropagation delay (T1) and the time difference value (X), a second timeadvance value (TA2) for use in transmitting signals from the user deviceto the target BS. For example, TA2 may be determined or calculated asTA2=2*(T1−X)=TA1−2*X. Performing, by the user device, a handover fromthe source BS to the target B. And, transmitting signals from the userdevice to the target BS based upon the second time advance value (TA2).

In an example implementation, the performing the handover may includeperforming a synchronized and random access-free handover from thesource BS to the target BS in response to a handover command receivedfrom the source BS, the synchronized handover being performed by theuser device at a time identified in the handover command.

In an example implementation, the performing a handover from the sourceBS to the target BS may include: sending, from the user device to thesource BS, a measurement report indicating a handover to the target BS;receiving, by the user device from the source BS, a handover commandincluding at least a handover time field that identifies a time toperform the handover to the target BS; beginning to receive data fromthe target BS at a time identified by the handover time field; andsending a handover completion message from the user device to the targetBS.

In an example implementation, the determining the second time advancevalue (TA2) may include determining the second time advance value basedon the following: TA2=TA1−2*X, wherein TA1 is the first time advancevalue, TA2 is the second time advance value, and X is the timedifference value.

In an example implementation, the determining the time difference value(X) may include the user device performing the following: receiving afirst signal from the source BS; receiving a second signal from thetarget BS, wherein the first signal and the second signal aretransmitted at the same time; and determining a difference in an arrivaltime between the first signal and the second signal as received by theuser device, wherein the time difference value includes the differencein arrival time of the first signal and the second signal.

In an example implementation, the determining the time difference valueincludes the user device performing the following: receiving a firstcommon reference signal(s) (CRS) from the source BS; receiving a secondCRS from the target BS, wherein the first CRS and the second CRS aretransmitted at the same time; and determining a difference in an arrivaltime between the first CRS and the second CRS as received by the userdevice, wherein the time difference value includes the difference inarrival time of the first CRS and the second CRS.

FIG. 6 is a diagram illustrating a second example implementation inwhich a target BS estimates a time advance value for a user device withrespect to the target BS. According to a second example implementationillustrated in FIG. 6, a passive scheme is provided where: At operation1, the source cell/source BS 132 informs the target cell/target BS 142about the configuration of the handover candidate (e.g., configurationof the user device 134), such as the information identifying the userdevice 134 (e.g., MSID or C-RNTI), source cell ID or source BS ID, SRS(sounding reference signal) configuration that identifies aconfiguration of the SRS signals transmitted by the user device 134, acurrent time advance value (e.g., TA1) used by the user device 134 whentransmitting to the source BS 132, and potentially also schedulinginformation (e.g., uplink resources scheduled by source BS 132 to allowthe user device 134 to transmit data or SRS signals to the source BS132). At operation 2A, the user device 134 may transmit signals (e.g.,SRS signals) via the scheduled resources. By knowing this information,at operation 2B, at the target cell/target BS 142 is able to measure theuplink (UL) signals (e.g., SRS signals) transmitted from the user device134/UE at operation 2A in FIG. 6, and hence estimate a timing advance(TA2) for the user device with respect to the target cell/target BS 142.Following, at operation 3), at the time of the handover, the target BS142 informs the user device 134 of the time advance value (TA2). Forexample, the target cell/target BS 142 may send the time advance value(TA2) for the target BS 142 to the user device 134/UE in one of thefirst downlink (DL) transmissions after the user device starts to listen(e.g., receive and decode) to signals from the target cell/target BS142. For example, the time advance value (e.g., TA2) with respect to thetarget BS 142 may be sent by target BS 142 to user device 134 via ascheduling grant sent to the user device 134. At operation 4) of FIG. 6,the user device may transmit data to the target BS based (e.g., byadvancing the start of transmission by TA2) on the time advance value(TA2).

According to an example implementation, a target BS may determine a timeadvance value for a user device with respect to the target BS based onthe following: Receiving, by a target base station (BS) from a sourceBS, information identifying a user device that is connected to thesource BS, information identifying a source cell or the source BS, and afirst time advance value used by the user device to transmit signals tothe source BS; receiving a signal (e.g., SRS signals) by the target BSthat was transmitted from the user device based on the first timeadvance value; determining, by the target BS based upon the first timeadvance value and the received signal from the user device, a secondtime advance value to be used by the user device to transmit data to thetarget BS; sending the second time advance value from the target BS tothe user device; receiving a handover of the user device from the sourceBS to the target BS; and receiving data by the target BS from the userdevice based on the second time advance value.

The technique may further include receiving by the target BS from thesource BS a handover request to perform a handover of the user devicefrom the source BS to the target BS, the handover request including ahandover time that identifies a time when a synchronized handover forthe user device is to be performed from the source BS to the target BS.

The example technique may further include receiving, by the target BSfrom the source BS, a signal configuration information identifying aconfiguration of a reference signal transmitted by the user device tothe source BS using the first time advance value; receiving, by thetarget BS from the source BS, scheduling information identifyingscheduled resources for the user device to transmit the reference signalto the source BS; and, wherein the receiving a signal by the target BSfrom the user device includes the target BS receiving the scheduledreference signal from the user device.

In an example implementation, the determining may include determining,by the target BS, a second time advance value to be used by the userdevice to transmit data to the target BS based upon the first timeadvance value and the reference signal received from the user device viathe scheduled resources.

In an example implementation, the receiving a handover may includereceiving a synchronized and random access-free handover of the userdevice from the source BS to the target BS, the synchronized handoverbeing performed at a time identified by a handover command.

FIG. 7 is a diagram illustrating a third example implementation in whicha source BS determines a time advance value for a user device withrespect to a target BS. This third example implementation (summarized inFIG. 7) is similar to the first example implementation, with at leastone difference here including that the source BS/source cell determinesa TA (e.g., TA2) for the target cell based on measurements reported fromthe user device/UE. At operation 1) in FIG. 7, the source cell/source BS132 instructs the user device 134 or UE to measure target cell timing.For example, the source BS 132 may instruct the user device to measurethe target BS propagation delay (e.g., T2) from the target BS 142 to theuser device 134.

At operation 2 of FIG. 7, the user device 134 or UE may measure thetarget BS propagation delay, e.g., based on the delay of commonreference signals (CRS) transmitted from the target BS 142. At operation3) of FIG. 7, the user device 134 then sends this target BS propagationdelay (e.g., T2) to the source cell/source BS 132. At operation 4) ofFIG. 7, the source cell/source BS 142 determines a time advance value(e.g., TA2) for the user device with respect to the target BS (or fortransmitting to the target BS 142). This time advance value (TA2) may bedetermined, for example, based on the time advance value (TA1) for theuser device with respect to the source BS 132 and the target BSpropagation delay. Also, the source cell/source BS 132 may send ahandover command to the user device 134, including the time advancevalue (TA2) with respect to the target BS 142. At operation 5) of FIG.7, the user device 134 transmits data, e.g., after handover, to thetarget cell/target BS 142 based on the time advance value

The method or technique of FIG. 7 may be implemented according to thefollowing, for example: determining, by a source base station (BS), afirst time advance value for the user device to use in transmittingsignals from the user device to source BS; sending, from the source BSto the user device a request to measure a target BS propagation delay ofa signal transmitted from a target BS to the user device; receiving thetarget BS propagation delay from the user device; determining, by thesource BS, a second time advance value for the user device to use intransmitting signals to the target BS; and, sending a handover commandto the user device, the handover command including the second timeadvance value.

The method or technique of FIG. 7 may include one or more additional ordifferent operations. According to an example implementation, thedetermining, by the source BS, the second time advance value for theuser device may include determining, by the source BS based on the firsttime advance value and the target BS propagation delay, the second timeadvance value for the user device to use in transmitting signals to thetarget BS.

According to an example implementation, the determining, by the sourceBS, the second time advance value for the user device may includedetermining, by the source BS based on the following: TA2=TA1−2*(T2−T1),where T1 is a source BS propagation delay of a signal transmitted fromthe source BS to the user device, T2 is the target BS propagation delay,TA1 is the first time advance value for the user device, and TA2 is thesecond time advance value for the user device.

According to an example implementation, the technique of FIG. 7 mayfurther include performing a synchronized and random accessprocedure-free handover of the user device from the source BS to thetarget BS based on the handover command, the handover of the user devicebeing performed at a time identified in the handover command.

According to an example implementation, the technique of FIG. 7 mayfurther include performing a handover of the user device from the sourceBS to the target BS, including: receiving, by the source BS from theuser device, a measurement report indicating a handover to the targetBS; and sending, by the source BS to the user device, a handover commandincluding at least a handover time field that identifies a time toperform a random access procedure-free and synchronized handover to thetarget BS.

A fourth example implementation will be described with reference toFIGS. 8 and 9. FIG. 8 is a diagram illustrating decreasing a size of apacket to keep the packet within a boundary, such as within a packet orframe boundary. FIG. 9 is a diagram illustrating the decreasing of asize of a packet or TTI to avoid interference, e.g., to avoidinterference with an adjacent TTI, packet, frame or other chunk of data.In an example implementation with respect to both FIGS. 8 and 9, it maybe assumed that the user device does not yet have a time advance valuefor transmitting to the target BS. Therefore, as shown in FIG. 9, Tp isthe propagation delay for the target BS 142 to the user device 134. Inorder to shorten or decrease a size of a packet for transmission, theuser device 134 may delay the transmission at 910 by Tp, for example, orother time period. In addition, or in the alternative, to decrease apacket size, the user device 134 may prune or omit a portion of thepacket or TTI at 912, e.g., by omitting (at 912) the transmission of oneor more symbols at the beginning, middle or end of the packet.

According to an example implementation, the use device 134 may shortenits packet size an amount based on a coverage area (or expected coveragearea) or a transmission power of the target BS 142. For example, moresymbols of a packet may be omitted based on a larger coverage area orhigher transmission power. In some cases, the transmission power of thetarget BS may define the coverage area or expected coverage area of thetarget BS, e.g., a higher transmission power will typically result in agreater coverage area. A larger coverage area may also mean a widervariation in time advance values for user devices or UEs within suchcell or coverage area. By shortening a packet size based on target cellcoverage area or transmission power, this technique may allow a userdevice to safely transmit data in an uplink direction to a target BS,even though the user device does not yet have the time advance value fortransmitting to the target BS.

At operation 1) of FIG. 8, the source cell/source BS 132 sends to thetarget BS 142/target cell a request for an expected coverage range ortransmission power for the target BS/target cell. At operation 2) ofFIG. 8, the target BS 142 may send to the source BS 132 the expectedcoverage range or transmission power of the target BS 142/target cell.At operation 3) of FIG. 8, the source BS 132/source cell sends theexpected coverage area or transmission power for the target BS/targetcell to the user device 134 or UE. At operation 4) of FIG. 8, the targetBS 142 sends a message to the user device scheduling UL resources forthe user device to transmit to the target BS, even though in thisexample case, the user device may not have a time advance value withrespect to the target BS 142. At operation 5) of FIG. 8, the user device134 transmits data with a shortened TTI or shortened packet (e.g., basedon coverage are or transmission power of target BS/cell) via scheduledUL resources. For example, one or more symbols may be omitted, at thebeginning, middle or end (or any location) of a packet or TTI(transmission time interval). This technique may be used until userdevice 134 receives a time advance value from target BS. This techniquemay be combined with one or more other techniques described herein.Also, any of the techniques described herein may be combined.

The fourth example implementation may be implemented, for example,according to the following: sending, from a source base station (BS) toa target BS, a request for expected coverage range or transmission powerlevel for a cell associated with the target BS; receiving, by the sourceBS from the target BS, the expected coverage range or transmission powerlevel of the cell associated with the target BS; sending the expectedcoverage range or transmission power level of the cell associated withthe target BS from the source BS to a user device; sending a handovercommand to the user device to cause a handover for the user device fromthe source BS to the target BS, wherein the user device omits one ormore symbols from the transmitted data packet. The transmitted datapacket may be confined to one or more transmit time intervals (TTIs)transmitted by the user device to the target BS based on the expectedcoverage range or transmission power level of the target BS.

According to an example implementation, the user device omitting mayinclude the user device omitting a first number of symbols of one ormore packets if the expected coverage range or transmission power isabove a first threshold and less than a second threshold, and the userdevice omitting a second number of symbols of one or more packets if theexpected cover range or transmission power is greater than the secondthreshold, wherein the second number is greater than the first number.

According to an example implementation, the user device omitting mayinclude the user device omitting three symbols of one or more packets ifthe expected coverage range or transmission power indicates a macro cellor large cell; omitting two symbols of one or more packets if theexpected coverage range or transmission power indicates a micro cell ormedium cell, where the micro cell is smaller than the macro cell; and,omitting one symbol of one or more packets if the expected coveragerange or transmission power indicates a pico cell or small cell, wherethe pico cell is smaller than the micro cell.

FIG. 10 is a diagram illustrating a fifth example implementation inwhich a target BS estimates a time advance value for a user device withrespect to the target BS, and then forwards the time advance value tothe source BS for forwarding to the user device. This fifth exampleimplementation is very similar to the second example implementation,except in the fifth example implementation, the target BS sends thedetermined time advance value (TA2) to the source BS for forwarding tothe user device via a handover command.

According to a fifth example implementation illustrated in FIG. 10, analternative passive scheme is provided where: At operation 1, the sourcecell/source BS 132 informs the target cell/target BS 142 about theconfiguration of the handover candidate (e.g., configuration of the userdevice 134), such as the information identifying the user device 134(e.g., MSID or C-RNTI), source cell ID or source BS ID, SRS (soundingreference signal) configuration that identifies a configuration of theSRS signals transmitted by the user device 134, a current time advancevalue (e.g., TA1) used by the user device 134 when transmitting to thesource BS 132, and potentially also scheduling information (e.g., uplinkresources scheduled by source BS 132 to allow the user device 134 totransmit data or SRS signals to the source BS 132). At operation 2A ofFIG. 10, the user device 134 may transmit signals (e.g., SRS signals)via the scheduled resources. By knowing this information, at operation2B, at the target cell/target BS 142 is able to receive and measure theuplink (UL) signals (e.g., SRS signals) transmitted from the user device134/UE at operation 2A in FIG. 10, and hence the target BS 142 canestimate a timing advance (TA2) for the user device with respect to thetarget cell/target BS 142. Following, at operation 3) of FIG. 10, thetarget BS 142 sends the time advance value (TA2) for the target BS 142to the source BS 132. At operation 4) of FIG. 10, the source BS 132informs the user device 134 of the time advance value (TA2). Forexample, the target cell/target BS 142 may send the time advance value(TA2) for the target BS 142 to the user device 134/UE in one of thefirst downlink (DL) transmissions after the user device starts to listen(e.g., receive and decode) to signals from the target cell/target BS142. For example, the time advance value (e.g., TA2) with respect to thetarget BS 142 may be sent by target BS 142 to user device 134 via ascheduling grant sent to the user device 134. At operation 5) of FIG.10, the user device 134 may transmit data to the target BS 142 based(e.g., by advancing the start of transmission by TA2) on the timeadvance value (TA2).

According to an example implementation, a target BS may determine a timeadvance value for a user device with respect to the target BS based onthe following: receiving, by a target base station (BS) from a sourceBS, information identifying a user device that is connected to thesource BS, information identifying a source cell or the source BS, and afirst time advance value used by the user device to transmit signals tothe source BS; receiving a signal by the target BS that was transmittedfrom the user device based on the first time advance value; determining,by the target BS based upon the first time advance value and thereceived signal from the user device, a second time advance value to beused by the user device to transmit data to the target BS; sending thesecond time advance value from the target BS to the source BS;receiving, by the target BS, a handover of the user device from thesource BS to the target BS; and receiving data by the target BS from theuser device based on the second time advance value.

In an example implementation, the source BS sends the second timeadvance value to the user device.

The fifth implementation may further include the source BS sending ahandover command to the user device, the handover command identifying asynchronized and random access procedure-free handover to the target BS,a time to perform the handover, and the second time advance value.

The fifth implementation may further include receiving by the target BSfrom the source BS a handover request to perform a handover of the userdevice from the source BS to the target BS, the handover requestincluding a handover time that identifies a time when a synchronizedhandover for the user device is to be performed from the source BS tothe target BS; and sending, by the target BS to the source BS, ahandover request acknowledgement that acknowledges that a synchronizedhandover will be performed for the user device from the source BS to thetarget BS at the identified time.

The fifth implementation may further include receiving, by the target BSfrom the source BS, a signal configuration information identifying aconfiguration of a reference signal transmitted by the user device tothe source BS using the first time advance value; receiving, by thetarget BS from the source BS, scheduling information identifyingscheduled resources for the user device to transmit the reference signalto the source BS; wherein the receiving a signal by the target BS fromthe user device includes the target BS receiving the scheduled referencesignal from the user device.

In an example implementation, the determining may include determining,by the target BS, a second time advance value to be used by the userdevice to transmit data to the target BS based upon the first timeadvance value and the reference signal received from the user device viathe scheduled resources.

In an example implementation, the receiving a handover may includereceiving a synchronized and random access-free handover of the userdevice from the source BS to the target BS, the synchronized handoverbeing performed at a time identified by a handover command.

FIG. 11 is a flow chart illustrating operation of a user deviceaccording to an example implementation. Operation 1110 includesreceiving, by the user device from the source BS, a handover commandincluding at least a handover time field that identifies a time toperform a synchronized handover to the target BS. Operation 1120includes performing, by the user device without using a random accessprocedure, a handover from the source BS to the target BS at the timeidentified by the handover time field. And, operation 1130 includesending a handover completion message from the user device to the targetBS.

FIG. 12 is a flow chart illustrating operation of a user deviceaccording to another example implementation. Operation 1210 includesdetermining, by a user device, a first time advance value for the userdevice for use in transmitting signals from the user device to a sourcebase station (source BS). Operation 1220 includes determining, by theuser device, a first propagation delay from a source base station (BS)to the user device. Operation 1230 includes determining, by the userdevice, a time difference value as a difference between the firstpropagation delay and a second propagation delay, the second propagationdelay including a propagation delay from a target BS to the user device.Operation 1240 includes determining, by the user device based on thefirst propagation delay and the time difference value, a second timeadvance value for use in transmitting signals from the user device tothe target BS. Operation 1250 includes performing, by the user device, ahandover from the source BS to the target BS. Operation 1260 includestransmitting signals from the user device to the target BS based uponthe second time advance value.

FIG. 13 is a flow chart illustrating operation of a target base stationaccording to an example implementation. Operation 1310 includesreceiving, by a target base station (BS) from a source BS, informationidentifying a user device that is connected to the source BS,information identifying a source cell or the source BS, and a first timeadvance value used by the user device to transmit signals to the sourceBS. Operation 1320 includes receiving a signal by the target BS that wastransmitted from the user device based on the first time advance value.Operation 1330 includes determining, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS. Operation 1340 includes sending the second timeadvance value from the target BS to the user device. Operation 1350includes receiving a handover of the user device from the source BS tothe target BS. And, operation 1360 includes receiving data by the targetBS from the user device based on the second time advance value.

FIG. 14 is a flow chart illustrating operation of a target base stationaccording to another example implementation. Operation 1410 includesreceiving, by a target base station (BS) from a source BS, informationidentifying a user device that is connected to the source BS,information identifying a source cell or the source BS, and a first timeadvance value used by the user device to transmit signals to the sourceBS. Operation 1420 includes receiving a signal by the target BS that wastransmitted from the user device based on the first time advance value.Operation 1430 includes determining, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS. Operation 1440 includes sending the second timeadvance value from the target BS to the source BS. Operation 1450includes receiving, by the target BS, a handover of the user device fromthe source BS to the target BS. Operation 1460 includes receiving databy the target BS from the user device based on the second time advancevalue.

FIG. 15 is a flow chart illustrating operation of a source base stationaccording to an example implementation. Operation 1510 may includedetermining, by a source base station (BS), a first time advance valuefor the user device to use in transmitting signals from the user deviceto source BS. Operation 1520 includes sending, from the source BS to theuser device a request to measure a target BS propagation delay of asignal transmitted from a target BS to the user device. Operation 1530includes receiving the target BS propagation delay from the user device.Operation 1540 includes determining, by the source BS, a second timeadvance value for the user device to use in transmitting signals to thetarget BS. Operation 1550 includes sending a handover command to theuser device, the handover command including the second time advancevalue.

FIG. 16 is a flow chart illustrating operation of a source base stationaccording to an example implementation. Operation 1610 includes sending,from a source base station (BS) to a target BS, a request for expectedcoverage range or transmission power level for a cell associated withthe target BS. Operation 1620 includes receiving, by the source BS fromthe target BS, the expected coverage range or transmission power levelof the cell associated with the target BS. Operation 1630 includessending the expected coverage range or transmission power level of thecell associated with the target BS from the source BS to a user device.Operation 1640 includes sending a handover command to the user device tocause a handover for the user device from the source BS to the targetBS, wherein the user device omits one or more symbols from one or morepackets transmitted by the user device to the target BS based on theexpected coverage range or transmission power level of the target BS.

FIG. 17 is a block diagram of a wireless station (e.g., a BS or a userdevice, or other communications device) 1700 according to an exampleimplementation. The wireless station 1700 may include, for example, twoRF (radio frequency) or wireless transceivers 1702A, 1702B, where eachwireless transceiver includes a transmitter to transmit signals and areceiver to receive signals. An antenna may be provided for eachwireless transceiver. The wireless station also includes a processor1704 to execute instructions or software and control transmission andreceptions of signals, and a memory 1706 to store data and/orinstructions.

Also, multiple communications or network interfaces 1710 are provided toallow the wireless station 1700 to communicate via differentcommunications media. Processor 1704 may also make decisions ordeterminations, generate frames, packets or messages for transmission,decode received frames or messages for further processing, and othertasks or functions described herein. Processor 1704, which may be abaseband processor, for example, may generate messages, packets, framesor other signals for transmission via wireless transceiver 1702.Processor 1704 may control transmission of signals or messages over awireless network, and may receive signals or messages, etc., via awireless network (e.g., after being down-converted by wirelesstransceiver 1702, for example). Processor 1704 may be programmable andcapable of executing software or other instructions stored in memory oron other computer media to perform the various tasks and functionsdescribed above, such as one or more of the tasks or methods describedabove. Processor 1704 may be (or may include), for example, hardware,programmable logic, a programmable processor that executes software orfirmware, and/or any combination of these. Using other terminology,processor 1704 and transceiver 1702 together may be considered as awireless transmitter/receiver system, for example.

In addition, referring to FIG. 17, a controller (or processor) 1708 mayexecute software and instructions, and may provide overall control forthe station 1700, and may provide control for other systems not shown inFIG. 17, such as controlling input/output devices (e.g., display,keypad), and/or may execute software for one or more applications thatmay be provided on wireless station 1700, such as, for example, an emailprogram, audio/video applications, a word processor, a Voice over IPapplication, or other application or software.

In addition, a storage medium may be provided that includes storedinstructions, which when executed by a controller or processor mayresult in the processor 1704, or other controller or processor,performing one or more of the functions or tasks described above.

Implementations of the various techniques described herein may beimplemented in digital electronic circuitry, or in computer hardware,firmware, software, or in combinations of them. Implementations mayimplemented as a computer program product, i.e., a computer programtangibly embodied in an information carrier, e.g., in a machine-readablestorage device or in a propagated signal, for execution by, or tocontrol the operation of, a data processing apparatus, e.g., aprogrammable processor, a computer, or multiple computers. A computerprogram, such as the computer program(s) described above, can be writtenin any form of programming language, including compiled or interpretedlanguages, and can be deployed in any form, including as a stand-aloneprogram or as a module, component, subroutine, or other unit suitablefor use in a computing environment. A computer program can be deployedto be executed on one computer or on multiple computers at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

Method steps may be performed by one or more programmable processorsexecuting a computer program to perform functions by operating on inputdata and generating output. Method steps also may be performed by, andan apparatus may be implemented as, special purpose logic circuitry,e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. Elements of a computer may include atleast one processor for executing instructions and one or more memorydevices for storing instructions and data. Generally, a computer alsomay include, or be operatively coupled to receive data from or transferdata to, or both, one or more mass storage devices for storing data,e.g., magnetic, magneto-optical disks, or optical disks. Informationcarriers suitable for embodying computer program instructions and datainclude all forms of non-volatile memory, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor andthe memory may be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, implementations may beimplemented on a computer having a display device, e.g., a cathode raytube (CRT) or liquid crystal display (LCD) monitor, for displayinginformation to the user and a keyboard and a pointing device, e.g., amouse or a trackball, by which the user can provide input to thecomputer. Other kinds of devices can be used to provide for interactionwith a user as well; for example, feedback provided to the user can beany form of sensory feedback, e.g., visual feedback, auditory feedback,or tactile feedback; and input from the user can be received in anyform, including acoustic, speech, or tactile input.

Implementations may be implemented in a computing system that includes aback-end component, e.g., as a data server, or that includes amiddleware component, e.g., an application server, or that includes afront-end component, e.g., a client computer having a graphical userinterface or a Web browser through which a user can interact with animplementation, or any combination of such back-end, middleware, orfront-end components. Components may be interconnected by any form ormedium of digital data communication, e.g., a communication network.Examples of communication networks include a local area network (LAN)and a wide area network (WAN), e.g., the Internet.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the various embodiments.

1-47. (canceled)
 48. An apparatus comprising at least one processor andat least one memory including computer instructions, when executed bythe at least one processor, cause the apparatus to: determine, by a userdevice, a first time advance value for the user device for use intransmitting signals from the user device to a source base station(source BS); determine, by the user device, a first propagation delayfrom the source BS and the user device; determine, by the user device, atime difference value as a difference between the first propagationdelay and a second propagation delay, the second propagation delayincluding a propagation delay from a target base station (target BS) andthe user device; determine, by the user device based on the firstpropagation delay and the time difference value, a second time advancevalue for use in transmitting signals from the user device to the targetBS; perform, by the user device, a handover from the source BS to thetarget BS; and transmit signals from the user device to the target BSbased upon the second time advance value.
 49. The apparatus of claim 48,wherein the at least one memory and the computer program code arefurther configured to, with the at least one processor, cause theapparatus to perform a synchronized and random access-free handover fromthe source BS to the target BS in response to a handover commandreceived from the source BS, the synchronized handover being performedby the user device at a time identified in the handover command.
 50. Theapparatus of claim 48, wherein the at least one memory and the computerprogram code are further configured to, with the at least one processor,cause the apparatus to: send, from the user device to the source BS, ameasurement report indicating a handover to the target BS; receive, bythe user device from the source BS, a handover command including atleast a handover time field that identifies a time to perform thehandover to the target BS; begin to receive data from the target BS at atime identified by the handover time field; and send a handovercompletion message from the user device to the target BS.
 51. Theapparatus of claim 48, wherein the at least one memory and the computerprogram code are further configured to, with the at least one processor,cause the apparatus to determine the second time advance value based onthe following: TA2=TA1−2*X, wherein TA1 is the first time advance value,TA2 is the second time advance value, and X is the time differencevalue.
 52. The apparatus of claim 48, wherein the at least one memoryand the computer program code are further configured to, with the atleast one processor, cause the apparatus to: receive a first signal fromthe source BS; receive a second signal from the target BS, wherein thefirst signal and the second signal are transmitted at the same time; anddetermine a difference in arrival time between the first signal and thesecond signal as received by the user device, wherein the timedifference value comprises the difference in arrival time of the firstsignal and the second signal.
 53. The apparatus of claim 48, wherein theat least one memory and the computer program code are further configuredto, with the at least one processor, cause the apparatus to: receive atleast one first common reference signal (CRS) from the source BS;receive at least one second CRS from the target BS, wherein the firstCRS and the second CRS are transmitted at the same time; and determine adifference in an arrival time between the least one first CRS and theleast one second CRS as received by the user device, wherein the timedifference value comprises the difference in arrival time of the leastone first CRS and the at least one second CRS.
 54. An apparatuscomprising at least one processor and at least one memory includingcomputer instructions, when executed by the at least one processor,cause the apparatus to: receive, by a target base station (target BS)from a source base station (source BS), information identifying a userdevice that is connected to the source BS, information identifying asource cell or the source BS, and a first time advance value used by theuser device to transmit signals to the source BS; receive a signal bythe target BS that was transmitted from the user device based on thefirst time advance value; determine, by the target BS based upon thefirst time advance value and the received signal from the user device, asecond time advance value to be used by the user device to transmit datato the target BS; send the second time advance value from the target BSto the user device; receive a handover of the user device from thesource BS to the target BS; and receive data by the target BS from theuser device based on the second time advance value.
 55. The apparatus ofclaim 54, wherein the at least one memory and the computer program codeare further configured to, with the at least one processor, cause theapparatus to receive by the target BS from the source BS a handoverrequest to perform a handover of the user device from the source BS tothe target BS, the handover request including a handover time thatidentifies a time when a synchronized handover for the user device is tobe performed from the source BS to the target BS.
 56. The apparatus ofclaim 54, wherein the at least one memory and the computer program codeare further configured to, with the at least one processor, cause theapparatus to: receive, by the target BS from the source BS, a signalconfiguration information identifying a configuration of a referencesignal transmitted by the user device to the source BS using the firsttime advance value; receive, by the target BS from the source BS,scheduling information identifying scheduled resources for the userdevice to transmit the reference signal to the source BS; wherein thereceiving a signal by the target BS from the user device includes thetarget BS receiving the scheduled reference signal from the user device.57. The apparatus of claim 56 wherein the at least one memory and thecomputer program code are further configured to, with the at least oneprocessor, cause the apparatus to determine, by the target BS, a secondtime advance value to be used by the user device to transmit data to thetarget BS based upon the first time advance value and the referencesignal received from the user device via the scheduled resources. 58.The apparatus of claim 56, wherein the reference signal comprises asounding reference signal.
 59. The apparatus of claim 54, wherein the atleast one memory and the computer program code are further configuredto, with the at least one processor, cause the apparatus to receive asynchronized and random access-free handover of the user device from thesource BS to the target BS in response, the synchronized handover beingperformed at a time identified by a handover command.
 60. An apparatuscomprising at least one processor and at least one memory includingcomputer instructions, when executed by the at least one processor,cause the apparatus to: determine, by a source base station (source BS),a first time advance value for a user device to use in transmittingsignals from the user device to the source BS; send, from the source BSto the user device a request to measure a target base station (targetBS) propagation delay of a signal transmitted from a target BS to theuser device; receive the target BS propagation delay from the userdevice; determine, by the source BS, a second time advance value for theuser device to use in transmitting signals to the target BS; send ahandover command to the user device, the handover command including thesecond time advance value.
 61. The apparatus of claim 60, wherein the atleast one memory and the computer program code are further configuredto, with the at least one processor, cause the apparatus to determine,by the source BS based on the first time advance value and the target BSpropagation delay, the second time advance value for the user device touse in transmitting signals to the target BS.
 62. The apparatus of claim60, wherein the at least one memory and the computer program code arefurther configured to, with the at least one processor, cause theapparatus to determine, by the source BS based on the following:TA2=TA1−2*(T2−T1), where T1 is a source BS propagation delay of a signaltransmitted from the source BS to the user device, T2 is the target BSpropagation delay, TA1 is the first time advance value for the userdevice, and TA2 is the second time advance value for the user device.63. The apparatus of claim 60, wherein the at least one memory and thecomputer program code are further configured to, with the at least oneprocessor, cause the apparatus to perform a synchronized and randomaccess procedure-free handover of the user device from the source BS tothe target BS based on the handover command, the handover of the userdevice being performed at a time identified in the handover command. 64.The apparatus of claim 60, wherein the at least one memory and thecomputer program code are further configured to, with the at least oneprocessor, cause the apparatus to perform a handover of the user devicefrom the source BS to the target BS, including: receive, by the sourceBS from the user device, a measurement report indicating a handover tothe target BS; and send, by the source BS to the user device, a handovercommand including at least a handover time field that identifies a timeto perform a random access procedure-free and synchronized handover tothe target BS.